Method for producing a heat-conducting connection between two work pieces

ABSTRACT

A method for producing a heat-conducting connection between two work pieces (1, 2) includes the steps of first producing a porous sintered layer (3), interposed between the two work pieces (1, 2) and sintered onto every work piece (1, 2) across a certain area, and subsequently compacting the porous sintered layer (3) sintered onto the two work pieces (1, 2) by pressing the two work pieces (1, 2) against each other.

CROSS REFERENCE TO RELATED APPLICATION

This is the 35 USC 371 National Stage of International ApplicationPCT/DE01/00932 filed on Mar. 12, 2001, which designated the UnitedStates of America.

FIELD OF THE INVENTION

The invention relates to a process for producing a heat-conductingconnection between two workpieces.

BACKGROUND OF THE INVENTION

For structures with highly heat-producing workpieces, for exampleworkpieces in the form of electrical or electronic components, goodheat-conducting connecting technology is necessary for heat dissipation.

Conventionally the components are soldered. The temperature stabilityand thermal fatigue resistance of solder is however very limited.

EP-0 242 626 A2 (GR 86 P 1242) discloses a process for producing aheat-conducting connection between a workpiece in the form of anelectronic component and a workpiece in the form of a substrate, inwhich a paste is applied to the component and/or the substrate; thepaste consists of a mixture of metal powder which sinters at thesintering temperature and sinters both to the component and also to thesubstrate, and of a liquid.

The paste is dried and after drying of the paste the component and thesubstrate are heated together or separately to a temperature which is atleast 100° C., but is still below the sintering temperature. Thisheating is expressly unpressurized heating according to the document.

At latest after this heating the component is placed on the completelydried paste of the substrate. The entire arrangement is then heated tothe sintering temperature with simultaneous application of mechanicalpressure of at least 900 N/cm².

This known process is especially suited for large-area, powersemiconductors which have been produced in MOS technology, majoradvantages however also being achieved in the production of otherelectronic components as well.

In the older German patent application 100 09 678.6 (GR 98 E 1895) whichhad not been published previously a process for producing aheat-conducting adhesive joint between two workpieces is proposed inwhich first of all a porous sintered layer of heat-conducting material,i.e. one which is penetrated with cavities like a sponge, especially ofsilver, is produced and is located between the two workpieces and issurface-sintered to each workpiece.

This sintered layer is filled with liquid hardenable cement whichafterwards wets each workpiece. Filling takes place for example bysucking the liquid cement into the cavities of the sintered layer whichact as capillaries.

Afterwards the cement is hardened and the adhesive joint is completed.

The sintered layer is produced such that a paste is applied to oneworkpiece and/or the other workpiece; the paste consists of a mixture ofa powder which sinters at the sintering temperature and which sinters toeach of the two workpieces at this temperature, and of a liquid; the twoworkpieces are brought together such that the paste is located betweenthe two workpieces and makes contact between the surfaces of the twoworkpieces, afterwards the paste is dried and the dried powder issintered by heating to the sintering temperature.

High density and thus good thermal conductivity of the sintered layer ofheat-conducting material can also be achieved with the step of applyinga certain mechanical pressure to the powder during the sintering processor after completion of this process.

Preferably a powder is used which is chosen from a group of metals,especially precious metals and semiprecious metals.

It is especially advantageous if a silver powder is used and sinteringof this powder is carried out in an oxidizing atmosphere, since tosinter this powder a sintering temperature between 100° C. and 250° C.is advantageously sufficient. Sintering in an oxidizing atmosphere canhowever also be advantageous for sinterable powders which containsubstances different from silver.

It is also mentioned that a high density and thus good thermalconductivity of the sintered layer of heat-conducting material can beachieved by applying a certain mechanical pressure to the powder duringthe sintering process or after completion of this process.

With the proposed process, larger workpieces can be advantageouslyjoined to surfaces to be joined which are larger than 1 cm², for example2×2 cm² or more, or even more, to one another securely over the entiresurface.

SUMMARY OF THE INVENTION

The object of the invention is make available a process for producing anadhesive-free, heat-conducting connection between two workpieces, withwhich larger workpieces which have surfaces to be joined which arelarger than 1 cm², for example 2×2 cm² or even more, can be connected toone another with high strength and very good heat conduction on thesesurfaces in their entirety.

This object is achieved by the features of claim 1.

According to this approach the process as claimed in the invention hasthe following steps:

Producing a porous sintered layer of heat-conducting material which islocated between the two workpieces and is sintered superficially to eachworkpiece, and

Subsequent compaction of the porous sintered layer which has beensintered to the two workpieces by pressing the two workpieces relativelyagainst one another.

The porous sintered layer of heat-conducting material, i.e. one which ispenetrated with cavities like a sponge, is produced preferably andadvantageously by the following steps:

Application of a paste to one workpiece and/or the other workpiece whichconsists of a mixture of a powder of heat-conducting material whichsinters at the sintering temperature and sinters to each of the twoworkpieces at this temperature, and a liquid,

Bringing together the two workpieces such that the paste is locatedbetween the two workpieces and makes surface contact between the twoworkpieces,

Drying of the paste, and

Sintering of the dried powder by heating to the sintering temperature.

Preferably and advantageously a powder of heat-conducting material isused which sinters at a sintering temperature of at most 250° C. andmoreover sinters to each of the two workpieces. Preferably a powder isused which is chosen from a group of metals, especially precious andsemiprecious metals, and which sinters at the sintering temperature ofat most 250° C. and moreover sinters to each of the two workpieces. Useof a sinterable metal powder which has silver is especiallyadvantageous.

It is especially advantageous if the sintering itself and sintering ofthe powder to each workpiece are carried out at the sinteringtemperature in an oxidizing atmosphere. This applies especially whenusing a sinterable metal powder which has silver.

For subsequent compaction of the porous sintered layer which has beensintered to the two workpieces it is advantageous to use mechanicalpressure which is chosen to be so high that as many cavities and poresof the porous sintered layer as possible, best all of them, are closed,but the two workpieces are not damaged.

It is especially advantageous if the subsequent compaction of the poroussintered layer which has been sintered to the two workpieces is carriedout by pressure sintering. Pressure sintering here means applying themechanical pressure used for subsequent compaction to the poroussintered layer with simultaneous heating of the porous sintered layer tothe sintering temperature which can be the same or different from thesintering temperature used in the production of the porous sinteredlayer.

Pressure sintering has the advantage that the pores of the sinteredlayer close more easily and the strength of the connection produced bysintering together between the sintered layer and the workpieces can beincreased even more.

For later compaction of the porous sintered layer which has beensintered to the two workpieces, feasibly and especially in pressuresintering a pressure of at least 900 N/cm² is used. An increase of thispressure to 1000 N/cm² or to 1500 cm² and more can be advantageous.

In the process as claimed in the invention, by producing the poroussintered layer which has been sintered to the workpieces a connectionbetween the workpieces is produced which is strengthened by thesubsequent compaction of the sintered layer especially by pressuresintering such that a high-strength, very good heat-conductingconnection between the workpieces is formed, which can have a largearea, especially larger than 2×2 cm², and is especially well suited forattachment of electronic components, especially power semiconductorcomponents such as for example IGBTs, MOS-FETs, diodes, thyristors,etc., which during operation produce high power losses which must beefficiently dissipated in order not to exceed the maximum operatingtemperature.

The process as claimed in the invention moreover has other majoradvantages especially compared to the process known from EP-0 242 626A2, including:

a) Simple positioning and fixing of a host of parts on a substrate, forexample for multichip modules, power converters or the like;

b) Surface irregularities are filled and cause fewer problems, forexample segregations on chips or roughness of the substrate. Thicknessfluctuations and irregularities due to screen pressure, templatepressure or spraying of metal—especially a silver layer—are eliminatedand the density of the pressed sintered layer is more homogeneous.

c) At a lower sintering temperature the stiffness of the sinter bridgesformed first in the sintered layer is still low according to the processas claimed in the invention, in this way at the same sintering pressurea higher compaction is achieved than in pressing the silver layer whichhas been sintered scratch-proof according to EP-0 242 626 A2 (see alsothe dissertation of Sven Klaka: “A low temperature connecting technologyfor building power semiconductor modules”, Cuvillier Verlag, Goettingen1997). The “softer” sintered layer according to the process as claimedin the invention is protected against damage by the workpieces betweenwhich it is located.

d) The sintering temperature used in the production of the sinteredlayer of the process as claimed in the invention essentially determinesthe connection temperature at which the workpieces are connected to oneanother without the mechanical tension due to different coefficients ofthermal expansion. It can remain clearly below the sintering temperaturewhich is used in pressure sintering.

The invention is detailed by way of example in the following descriptionusing the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in cross section two separate workpieces, to which onepaste at a time which consists of a mixture of a powder which sinters atthe sintering temperature and sinters to each of the two workpieces atthis temperature, and of a liquid, is applied,

FIG. 2 shows the workpieces as shown in FIG. 1 in the same view but inthe assembled state such that the paste forms a single continuous layerbetween the workpieces which brings the two workpieces into contact,

FIG. 3 shows the workpieces as shown in FIG. 2 in the same view, butafter drying of the paste and sintering of the powder of thermallyconductive material into a porous sintered layer which is locatedbetween the workpieces and brings the surfaces of the two workpiecesinto contact with one another,

FIG. 4 shows a sector A surrounded in a circle shape in FIG. 3 in anenlargement,

FIG. 5 shows the sector A as shown in FIG. 4 after compaction of theporous sintered layer, and the figures are schematic and not to scale.

DETAILED DESCRIPTION OF THE DRAWINGS

The heat-conducting connection as claimed in the invention between thetwo workpieces is detailed using the example of one preferred specialproduction process.

FIG. 1 shows as the initial stage of this process two workpieces 1 and 2which are separate from one another and which have surface sections 11and 21 which are opposite one another and are matched to one another interms of shape, for example are flat.

For example, let the workpiece 1 be an electronic component, for examplea power component, especially a power semiconductor component, and theworkpiece 2 be a carrier body to which the electronic component is to beattached.

A paste 5 which consists of a mixture of a powder of heat-conductingmaterial which sinters at the sintering temperature and sinters to eachof the two workpieces at this temperature, and of a liquid, is appliedto the surface section 21 of the workpiece 2 and/or to the surfacesection 11 of the workpiece 1. In FIG. 1 the paste 5 is shown applied toeach workpiece 1 and 2, but it is sufficient to apply the paste 5 onlyto one workpiece, for example the workpiece 2.

The two workpieces 1 and 2 are brought together after applying the paste5 such that the paste 5 is located between the two workpieces 1 and 2and the paste 5 makes contact between the surface section 11 and 21 ofeach workpiece 1 and 2 over as large an area as possible and forms athin layer 3′ between these sections 11 and 21, after which theintermediate stage of the process shown in FIG. 2 is formed.

Afterwards the layer 3′ of paste 5 is dried and sintered after heatingto the sintering temperature T.

For drying the paste 5 it is advantageous if the two joined workpieces 1and 2 are pressed against one another so that the paste 5 is squeezedout of at least one workpiece, for example the workpiece 1, in a smallbead 51, which surrounds this workpiece.

Drying of the paste 5 takes place for example by evaporation of theliquid contained in the paste 5, which can be done by heating the paste5, for example during heating to the sintering temperature T and/or at anegative pressure, for example in a vacuum. The bead 51 advantageouslycontributes to the liquid being evaporated without residue and withoutbubble formation.

After sintering of the dried powder at the sintering temperature T theintermediate stage of the process shown in FIG. 3 is formed.

This intermediate stage has a porous sintered layer 3 of dried powderwhich is located between the surface sections 11 and 21 of theworkpieces 1 and 2 and which has two flat-sided surfaces 31, 31 facingaway from one another and a bead 30 which surrounds at least oneworkpiece and which is formed from the bead 51.

One of the flat-sided surfaces 31, 31 borders the surface section 11 ofthe workpiece 1 over the entire surface, the other borders the surfacesection 21 of the workpiece 2 over the entire surface.

To increase the density of the porous sintered layer 3, during sinteringmechanical pressure p can be applied to the powder, but it must remainso low that the sintered layer 3 remains porous.

The sintering temperature T is determined by the powder material.

The section A of FIG. 3 shown enlarged in FIG. 4 shows the internalstructure of the sintered layer 3 by way of example and schematically.

In FIG. 4 the obliquely cross-hatched part 34 of the sintered layer 3contains sintered powder or heat-conducting material which is coherentfrom one flat-sided surface 31 in the direction 25 to the otherflat-sided surface 31 of the layer 3.

All non-crosshatched white areas 32 of the layer 3 represent cavities orpores of the layer 3. Although all these white areas would each have tobe provided with reference numbers 32, for the sake of clarity only afew of these areas are labeled with this reference number 32.

The cavities or pores 32 penetrate the layer 3 like a sponge and are forthe most part connected to one another, even if not in the cutting planeshown. Cavities 32 which border the flat-sided surfaces 31 each defineone opening 33 in this surface 31.

For the process described so far all workpieces and materials given inEP 0 242 626 A2 and older German patent application 100 09 678.6, theliquid of the paste and the powder of the paste and the sinteringtemperatures and pressures given there can be used. The entiredisclosure of EP 0 242 626 A2 and patent application 100 09 678.6 is acomponent of this application.

A sintered layer 3 of silver powder is especially suitable, since silverat low temperatures between 100° C. and 250° C., preferably between 150°C. and 250° C., can form silver bridges.

For example, suitable fine-grain silver powders are mixed for examplewith organic liquid, for example terpineol or ethylene glycol ether intoa paste 5 which can be processed like a conductive cement paste.

After applying the paste 5, for example with a dispenser, to at leastone of the two workpieces 1 or 2, which is for example a carrier bodyfor an electronic component in the form of a chip, the other workpiece 2or 1, in the example the chip, is placed on the paste 5 such that itsqueezes out peripherally in a small bead 51. Thus, when the paste 5 isslowly heated the liquid can be evaporated without residue and withoutbubble formation and the paste 5 dried.

After drying, between the workpieces 1 and 2 a layer 3 and a bead 30 ofdry silver powder which are sintered is formed.

For sintering of silver at less than 250° C. an oxidizing, preferablyoxygen-containing atmosphere is essential. Surprisingly, in the thinlayer 3 of silver powder of less than 100 microns between the workpieces1 and 2 the oxygen can diffuse rather quickly so that sintering of thesilver powder can take place in areas of up to 5×5 cm² or more. Forexample, in areas of 2×2 cm² sintering of the silver powder takes placewithin roughly 15 minutes.

It was found that silver powder in an O₂-containing atmosphere, forexample in air, begins to sinter surprisingly at low temperaturesbeginning from 150° C. The sintering process in this case arises by thesilver powder's consolidating into a sponge which has cavities or poresand acquiring a conspicuous adhesion capacity. For example, a hottweezer tip at low pressure adheres to the consolidated sponge of silverpowder. On many smooth surfaces such as for example silicon, glass,corundum, polyimide this adhesion occurs and it is strong enough tosinter for example a chip on glass and to cool it to room temperature.It also applies to silver that polished surfaces are especially wellsuited for this purpose since the silver particles come into closecontact with the surface. At a high temperature this adhesive capacityrecedes again.

The sintered layer 3 of silver powder produced in this way is penetratedin a sponge-like manner by cavities or pores 32 and on its flat-sidedsurfaces 31 has openings. The density of this layer 3, depending on theinitial powder, is between 40 to 50% by volume silver and can beincreased by adding much finer and also very much coarser powders.Instead of silver, also other substances with good thermal conductivity,but a low thermal expansion coefficient, such as for example SiC ordiamond, can be used as coarse-grain powders for example to better matchthe coefficient of thermal expansion of the sintered layer 3 of silverpowder to a chip.

A high silver density and thus good thermal conductivity can be achievedby using pressure at 150° C. to 250° C., and the pressure and timeshould remain far lower than in the conventional process according to EP0 242 626 A2 so that the sintered layer 3 remains porous.

The sintered layer 3 imparts to the connection of the two workpieces 1and 2 a certain strength.

Only now is the porous sintered layer 3 which has been sintered to thetwo workpieces 1 and 2 compacted by pressing the two workpieces 1 and 2against one another in relative terms.

This can be carried out by placing the workpieces 1 and 2 which areconnected by the porous sintered layer 3 between the dies 61 and 62 of apress which is generally labelled 6 and which is indicated in FIG. 4, asfollows for example from EP 0 242 626 A2 and is used for pressuresintering in the process there, and which exerts mechanical pressure Pon the sintered layer 3 via the workpieces 1 and 2.

Mechanical pressure P is used which is so high that as many cavities andpores 32 of the sintered layer 3 as possible, best all of them, areclosed, but the two workpieces 1 and 2 are not damaged.

FIG. 5 shows the sintered layer 3 which is located between theworkpieces 1 and 2 and which has been sintered to each of them, whichwas porous beforehand and now for example no longer has visible pores,after this compaction.

The strength of the connection can be increased if the subsequentcompaction of the porous sintered layer 3 which has been sintered to thetwo workpieces 1 and 2 is carried out by pressure sintering.

The pressure P which is used for subsequent compaction of the poroussintered layer 3 which has been sintered to the two workpieces 1 and 2should be at least 900 N/cm² and can be for example 1000 N/cm² or 1500N/cm² and more.

What is claimed is:
 1. Process for producing a heat conductingconnection between two workpieces (1, 2), which comprises the steps of:producing a porous sintered layer (3) of heat-conducting material whichis located between the two workpieces (1, 2) and is sinteredsuperficially to each workpiece (1, 2), and subsequently compacting theporous sintered layer (3) which has been sintered to the two workpieces(1, 2) by pressing the two workpieces relatively against one another (1,2), wherein the step for producing the porous sintered layer (3)includes the steps of: applying a paste (5) to at least one workpiece(1; 2), said paste comprising a mixture of a powder of heat-conductingmaterial which sinters at a sintering temperature (T) to each of the twoworkpieces (1, 2), and a liquid, said powder being selected from a groupof precious metals and semiprecious metals, bringing together the twoworkpieces (1, 2) such that the paste (5) is located between the twoworkpieces (1, 2) and makes surface contact between the two workpieces(1, 2), drying the paste (5), and sintering the dried powder by heatingto the sintering temperature (T).
 2. Process as claimed in claim 1,wherein the powder of heat-conducting material sinters at a sinteringtemperature (T) of at most 250° C.
 3. Process as claimed in claim 1,wherein the powder is a sinterable metal powder which contains silver.4. Process as claimed in claim 1, wherein the sintering itself andsintering of the powder to each workpiece (1, 2) are carried out at thesintering temperature (T) in an oxidizing atmosphere.
 5. Process asclaimed in claim 1, wherein the compacting step is carried out withmechanical pressure (P) which is so high that the pores (32) of thesintered layer (3) are closed, but the two workpieces (1, 2) are notdamaged.
 6. Process as claimed in claim 1, wherein the compacting stepis carried out by pressure sintering.
 7. Process as claimed in claim 1,wherein the compacting step is carried out at a pressure of at least 900N/cm².
 8. Process as claimed in claim 2, wherein the sintering itselfand sintering of the powder to each workpiece (1, 2) are carried out atthe sintering temperature (T) in an oxidizing atmosphere.
 9. Process asclaimed in claim 3, wherein the sintering itself and sintering of thepowder to each workpiece (1, 2) are carried out at the sinteringtemperature (T) in an oxidizing atmosphere.
 10. Process as claimed inclaim 5, wherein the compacting step is carried out at a pressure of atleast 900 N/cm².
 11. Process as claimed in claim 6, wherein thecompacting step is carried out at a pressure of at least 900 N/cm². 12.Process as claimed in claim 8, wherein the compacting step is carriedout at a pressure of at least 900 N/cm².
 13. Process as claimed in claim9, wherein the compacting step is carried out at a pressure of at least900 N/cm².